Increase visibility of sidebar icons in dark mode

[iccir]

The use of highly-saturated colors over a dark gray background is a longstanding problem with dark themes. It's often hard to distinguish the boundary of the two colors. This affects several of the file icons:

A common solution is to add an inner shadow or border to help with visibility:

The colors can also have their saturation reduced and/or lightness increased to further help:

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Unfortunately, Sublime Text has no easy way of specifying different icons for dark mode. Timeless Dark would need to be moved to its own package.

What we can do is:

1. Create a base icon image without a border. Use this image for layer0.
2. Make a transparent rounded rect with a black inner shadow. Use this for layer1 in light mode.
3. Make a transparent rounded rect with a white inner shadow. Use this for layer1 in dark mode.

Using this technique, we could also darken the outer shadow in dark mode (which could further increase visibility).

[iccir]

Unfortunately, due to Sublime Text Issue 6819, we can't currently use a separate layer with an outline - Sublime Text is sometimes drawing the file icon over the outline.

[iccir]

The solution (for now) is to use an exaggerated/extra shadow in layer1. Since there is no overlap with the file icon, layer ordering doesn't matter.

Note that this extra layer can cause conflicts with AFileIcon icons, so we need to add a setting to turn this off.

[iccir]

For now, I have added three rules to Timeless Dark.sublime-theme to style icon_folder/icon_file_type.

Ultimately:

• These rules should reside in Timeless.sublime-theme.
• Both light and dark variants should use an outline file for layer1 (as described above).
• The specific file used should be specified by a variable.